fix: dev ui url works in sub-apps

[racinmat]

Fixes #2072.
Blocked by #2251, it depends on it because of the base_url.
I didn't figure out any better way to do this. Basically we need to have a way how to provide a custom URL to the assets/config/runtime-config.json. I have tested with my project that it works well.

I have added test that demonstrates the functionality in a sub-app.

I ran the relevant tests to my code and here are the results, everything passes.

C:\Projects\others\adk-python ❯ pytest ./tests/unittests/cli
=========================================================================================== test session starts ===========================================================================================
platform win32 -- Python 3.12.8, pytest-8.4.1, pluggy-1.6.0
rootdir: C:\Projects\others\adk-python
configfile: pyproject.toml
plugins: anyio-4.9.0, langsmith-0.3.45, asyncio-1.1.0, mock-3.14.1, xdist-3.8.0
asyncio: mode=Mode.AUTO, asyncio_default_fixture_loop_scope=function, asyncio_default_test_loop_scope=function
collected 206 items

tests\unittests\cli\test_fast_api.py ....................................                                                                                                                            [ 17%]
tests\unittests\cli\utils\test_agent_loader.py ..................................                                                                                                                    [ 33%]
tests\unittests\cli\utils\test_cli.py ........                                                                                                                                                       [ 37%]
tests\unittests\cli\utils\test_cli_create.py ..................................                                                                                                                      [ 54%]
tests\unittests\cli\utils\test_cli_deploy.py ..........................................                                                                                                              [ 74%]
tests\unittests\cli\utils\test_cli_tools_click.py ..........................                                                                                                                         [ 87%]
tests\unittests\cli\utils\test_evals.py ..........................                                                                                                                                   [100%]